A natural resource such as oil or gas residing in a subterranean formation can be recovered by drilling a well into the formation. The subterranean formation is usually isolated from other formations using a technique known as well cementing. In particular, a wellbore is typically drilled down to the subterranean formation while circulating a drilling fluid through the wellbore. After the drilling is terminated, a string of pipe, e.g., casing, is run in the wellbore. Primary cementing is then usually performed whereby a cement slurry is pumped down through the string of pipe and into the annulus between the string of pipe and the walls of the wellbore to allow the cement slurry to set into an impermeable cement column and thereby seal the annulus. Secondary cementing operations may also be performed after the primary cementing operation. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is forced under pressure to areas of lost integrity in the annulus to seal off those areas.
Phosphate-based cementitious compositions are commonly used in primary and secondary cementing operations to form quick setting, strong cement that binds well to the subterranean formation and to itself. The phosphate-based cementitious compositions may be formed by combining together the following reactants: a metal oxide or hydroxide compound such as magnesium oxide; a phosphate compound such as potassium hydrophosphate; and water. The resulting reaction is fast such that the cementitious compositions set very rapidly, e.g., within a few minutes at room temperature and pressure. The rate of reaction further increases as the temperature increases. As such, the thickening times of the compositions may be unacceptably short to allow them to be pumped to their desired downhole locations, making the use of such compositions in well cementing a challenge. As a result, the drill pipe or the tool used to lower the piping in the wellbore may be cemented in place, causing several weeks of delay in the completion of the wellbore.
One method commonly employed to lengthen the thickening time of phosphate-based cementitious compositions is to introduce boric acid or salts thereof to the compositions, thereby delaying the rate of reaction. As a result, the thickening time of the phosphate-based cementitious compositions may endure for as long as 3 to 4 hours when used in a wellbore having a relatively low bottom hole static temperature. However, at higher temperatures, the thickening time of the phosphate-based cementitious compositions is still insufficient to allow for proper placement downhole. A need therefore exists to develop methods of reducing the thickening time of such phosphate-based cementitious compositions when they are exposed to relatively high downhole temperatures and pressures.